Investigation, Experimental Development, and Field Application of Non-Invasive Sensors for Partial Discharge Measurement

Electrical insulating systems represent one of the weakest elements of the entire electrical equipment. In fact, the lifetime of any electrical device depends on the capability of its insulation to last in time. In general, all organic materials, and in particular all dielectrics, are subjected to ageing. The ageing process is caused by different stresses: thermal, electrical, mechanical and environmental. By focusing on the electrical stress, it is possible to state that it is fundamentally characterized by two phenomena: the presence of an electric field and/or the presence of partial discharges (PDs), a phenomenon in which the interelectrode zone is partially short-circuited causing degradation of the material. It is clear, therefore, how partial discharges are both the symptom and the cause of the deterioration of the insulating materials. Thus, the measurement of this phenomenon leads to obtain an “image” of the status of the insulating system allowing to perform a predictive diagnostic by an expert user or by sophisticate artificial intelligence algorithms. These diagnostic techniques lead to reduce the maintenance costs since they allow to perform a condition-based maintenance that is carried out only when this is necessary, according to the real state of the component. To measure the partial discharge activity there are different methods, but it is possible to classify them into two groups: electrical detection methods and non-electrical detection methods. If the electrical methods typically need invasive procedures to perform the partial discharge measurement, the non-electrical ones, on the contrary, can be implemented on the electrical component without any change (non-invasive) in the normal operating of the equipment. The focus of this research activity is to study, develop and optimize prototypes, test them in laboratory and in real applications of different non-invasive sensors in order to measure partial discharges. Two kind of sensors are analysed: Transient Earth Voltage Sensors and High Frequency Current Transformers ones. Transient Earth Voltage Sensors are based on the homonymous operating principle, and they can be used when the electrical apparatus is placed inside a metal box like switchgears or gas insulated systems. Starting from a first investigation on the effect of the geometrical dimensions of different sensors prototype in order to maximize the signal output, a first preliminary on-field test campaign is performed on an industrial air insulated medium voltage switchgear. Then, the entire partial discharge measurement system configuration, based on a definitive version of TEV sensor, is optimized and tested on the industrial test set-up, by comparing the obtained PD pattern with those collected from an IEC 60270 standard compliant measurement system. The results suggest how the non-invasive PD measurement system provides information comparable to those collected by the standard-compliant one. The High Frequency Current Transformer Sensors are studied and optimized during an experimental laboratory activity where different magnetic core material and different sensor configurations are tested. The results allow to implement the HFCT sensor on an optimized PD measurement system introduced to perform a research investigation on the phenomenon of electrical treeing on HVDC cable insulation, carried out during an exchange program at the RWTH Aachen University in Germany. The aim of this investigation is to understand the behaviour of the electrical treeing with different voltage frequencies and amplitudes during long-term tests since some preliminary tests give unexpected results. In fact, it is generally accepted that increasing the frequency leads to an accelerated ageing process, mainly due to partial discharge activity, and therefore, to a reduced time to breakdown. The collected results show that increasing the applied voltage frequency, the time to breakdown increases since the electrical treeing shape changes. During this analysis, PD measurement is carried out in order to monitor the electrical treeing evolution during the degradation process by adopting the optimized HFCT-based measurement system.